At least this is my best guess based on the evidence I have so far.
Martin is walking around the community snooping at the condos. As a guy who has previously committed theft (the suspension in which he was caught with stolen girls’ jewelry and a screwdriver he presumably used to pry open lockers), he’s thinking about all of the criminal possibilities.
Then he notices some guy in a car tailing him. This is not the first time that people have found him suspicious. In fact, it’s a typical reaction from security guards, shopkeepers, and various regular middle-class people. If anything, Trayvon delights in intimidating middle-class people, because it give him a feeling of power. That’s why he purposefully cultivates a threatening demeanor which includes tattoos, gold teeth, a hoodie, and a badass attitude.
So when he notices he’s being tailed, he has a guilty mind and therefore runs away. But after running away, he feels angry at himself for such a cowardly act. He’s the badass Trayvon Martin, how dare some middle-class white Hispanic guy in a car make him run away like a girl. He is determined to go find the guy and teach him a lesson for looking at him the wrong way.
“Yo, why you be follin’ me?” Trayvon asks threateningly.
George Zimmerman is a little bit scared now. He tries to put up a bold front by asking “What are you doing around here?”
Travyon smells his fear, and then he attacks. We know the rest of the story based on previous blog posts about the evidence. One punch to his face breaks George’s nose, then Trayvon tackles him and bashes his head against the ground.
* * *
By the way, if you read the police report, you will see that the cops approached Zimmerman with guns drawn, and that Zimmerman was then handcuffed and brought back to the police station and questioned there, so the notion that he wasn't "arrested" because the cops are racist is merely semantics. The police chose not to call it an arrest after they determined that there was overwhelming evidence in favor of self-defense.
The police also held onto his gun as evidence.
Also, note that the cop states he heard Zimmerman say "I was yelling for someone to help me, but no one would help me." If someone had just come to help, Trayvon would still be alive and be yet another black youth charged with felony assault. But people, wisely self-interested, decided not to get any more involved than calling 911.
UPDATE, NEW INFO
There’s some additional evidence posted at the Orlando Sentinel last evening which I missed:
Zimmerman told them he lost sight of Trayvon and was walking back to his SUV when Trayvon approached him from the left rear, and they exchanged words.
Trayvon asked Zimmerman if he had a problem. Zimmerman said no and reached for his cell phone, he told police. Trayvon then said, "Well, you do now" or something similar and punched Zimmerman in the nose, according to the account he gave police.
Zimmerman fell to the ground and Trayvon got on top of him and began slamming his head into the sidewalk, he told police.
Zimmerman began yelling for help.
I think this somewhat contradicts Trayvon’s girlfriend’s account, but in both accounts they exchange words and then there’s a fight, no doubt started by Trayvon.
One witness, who has since talked to local television news reporters, told police he saw Zimmerman on the ground with Trayvon on top, pounding him — and was unequivocal that it was Zimmerman who was crying for help.
Zimmerman then shot Trayvon once in the chest at very close range, according to authorities.
When police arrived less than two minutes later, Zimmerman was bleeding from the nose, had a swollen lip and had bloody lacerations to the back of his head.
The key new info here is that the shot was at "very close range" and in the chest, which adds further evidence to my suspicion that Zimmerman did not have the opportunity to run away and that the "stand your ground" law is irrelevant and this would be self defense in any jurisdiction. Not that a shorter out-of-shape overweight injured man had much chance running away from a much taller younger guy who plays football.